How to Fix a Lawn Mower Carburetor When the Primer is Not Priming

Many models of mowers require priming before a cold start.

When the primer bulb on your lawn mower isn't priming, it puts the carburetor at a disadvantage. Designed to replace air in the fuel lines and carburetor with gasoline when you depress the button several times, a primer bulb provides a crucial service that makes starting the engine easier. Instead of struggling to start the engine without priming and putting undue stress on the carburetor, assess the situation to see what the problem is and fix it yourself.

Faulty Priming Symptoms

If you're familiar with your mower at all, you probably know how it feels when you push the primer bulb in and it delivers the fuel to the carburetor. There's a slight resistance when you push it, and you can feel that it's pulling fuel from the gas tank. When it's not working, however, the bulb may refuse to pop back out after you depress it. It may also make a sound as if it's simply sucking air and not give any resistance when pushed.

Possible Problems

When the primer bulb isn't working, it may be a problem with the bulb itself, with the fuel lines that feed fuel to the bulb or both. Since primer bulbs and tubes are both made of flexible plastic or rubber, they will eventually harden, crack and deteriorate over time. A primer bulb that's lost its suppleness doesn't respond as a new one does. The same goes for fuel lines. When they've hardened and cracked, they let in air, which makes it impossible to draw the fuel into the carburetor properly.

Primer Bulb Replacement

Primer bulbs are usually positioned on the lawn mower one of two ways. They're either directly on the carburetor, or placed at a distance from the carburetor and connected to it with rubber tubing. Once you've gained access to the bulb's housing, remove the screws that hold it in place. Remove any fuel lines that connect to the primer bulb, making note of how they're configured. Install the new primer bulb and reconnect any fuel lines as necessary. Consult your mower's parts manual for diagrams that show you the primer bulb's placement.

Fuel Line Replacement

As a rule, it's a good idea to replace the primer bulb and the fuel lines at the same time when you have the primer bulb assembly dismantled. Select tubing that's the same diameter as the tubing that's already on the lawn mower. Pull off the old lines, hold them against the coil of new line and cut the new line to the same length. Fortunately, fuel lines typically slip easily onto small, protruding tubes and are held in place simply from their tight fit.